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Royer Labs dBooster

Cascade Preamplifier
By Hugh Robjohns

Royer Labs dBooster

The idea of an in-line gain booster placed between a moving-coil or ribbon mic and a mic preamp isn't new — so what makes this one special?

Back in the '60s and '70s it was quite common to find professional mixing-console preamps offering 70dB of gain or more. Today, though, with the prevalence of high-output capacitor mics, relatively few contemporary preamps offer more than 60dB of preamp gain, and some even less. While this is fine for the most common applications, it can make it difficult to achieve a satisfactory recording level when working with, for example, spoken word into a Shure SM7b, or using a passive ribbon mic to capture a delicate sound source. A popular solution for such situations is the 'cascade preamp': a remote gain-stage device to boost a weak microphone signal before it reaches the interface or mixer's preamp. Today, there are many such devices from which to choose, with Cloud Microphones' Cloudlifter (reviewed in SOS November 2011) and the Triton Audio FetHead probably being the best known. Alternatives include the MartiAudio Booster (reviewed in SOS September 2016), the Radial McBoost, the Sanken HAD‑48, and the sE Electronics DM1 Dynamite (reviewed in SOS February 2019), amongst others.

All of these cascade preamps take their power from a standard 48V phantom supply that's provided by the main preamp and carried down the output cable, and most of them prevent that phantom power from being passed on to the connected microphone — a feature which often reassures ribbon mic users. The precise gain boost provided by cascade preamps like these typically varies depending on the preamp's input impedance, but most manage a useful 20-26 dB of gain.

In case you were wondering, the term 'cascade preamp' relates to the specific form of electronic engineering involved, where a (usually) very simple, fully balanced, discrete transistor circuit provides the gain boost. Some designs employ FETs as the active devices, others bi-polar junction transistors (BJTs), and others a combination of both — but the important thing is that this remote gain stage works directly in concert with the main preamp, and is effectively an extension of the preamp's own input circuitry.

Jolly Royer

Royer Labs dBoosterSince one of the most common applications for cascade preamps is in combination with passive ribbon microphones, it is understandable that Royer Labs — the American specialist ribbon mic manufacturers — have developed their own take on the concept. Although perhaps not (yet) particularly well known for their electronics, the company do already produce a number of active ribbon mics with built-in impedance-conversion and gain-boosting circuitry, so it's not a huge leap from there to Royer's latest product, the dBooster.

However, whereas most other manufacturers have gone down the 'less is more' route with their cascade preamp designs, generally employing simple, minimalist active circuitry, Royer's backroom boys have really gone to town with the design of the dBooster, which is extraordinarily sophisticated and elaborate. In fact, it wouldn't be too extreme to suggest the dBooster is virtually a full-fat low-gain mic preamp in its own right, rather than a simple cascade preamp and, as far as I'm aware, it's the only one currently on the market that features an internal regulated power supply, a discrete Class-A input gain stage with RF protection, and a separate actively balanced output line-driver built around a high-grade dual op-amp.

That discrete transistor input stage is unusually elaborate too, with eight — yes eight! — bi–polar junction transistors, arranged as four paralleled sets on each leg of the balanced input. This unique (for a cascade preamp) approach is designed to minimise input noise, justifying Royer's claim that the dBooster has "ultra-low noise and distortion". Moreover, because this input gain stage is isolated from the main mic preamp it's feeding by the line-driving op-amps, the gain does not vary with the preamp's input impedance, so the performance is consistent regardless of what it's partnered with. In addition, unlike most cascade preamps, which provide a fixed (one-level-suits-all) gain boost, the dBooster has a button on the input end of the unit which selects either 12 or 20 dB of gain. This has several benefits, including allowing greater headroom and lower noise when working with sources that don't need quite as much extra support.

There's rather more going on inside this box than in perhaps any other cascade preamplifier.There's rather more going on inside this box than in perhaps any other cascade preamplifier.Physically, Royer's dBooster is housed in a small, black-painted, extruded aluminium case with a non-slip rubber pad on the base, and it's all built in the USA. The dimensions are 41.5 x 51 x 92mm (1.6 x 2 x 3.6 inches) and it weighs just 170g (6oz). A male XLR3 on one end provides the output and receives standard 48V phantom power, from which it draws between 5 and 7.5 mA depending on the signal level. That's quite a lot compared to most of the competitors' devices, whose much simpler circuitry typically requires only a few milliamps. Still, the P48 phantom specification allows for a maximum of 10mA, so there shouldn't be a problem. At the opposite end of the box a female XLR3 accepts the microphone input, with that small square button I mentioned to configure the gain.

Letting my inner geek loose for a moment, I quickly removed a few screws to slide the circuitry from the case and was at first surprised — and then very impressed! A double-sided fibreglass PCB extends the full length and width of the case, and this carries an impressive assortment of SMT components on both sides. The eight-transistor input array is accommodated on the bottom of the board, while the output driver chip (a Texas Instruments OPA1692 dual op-amp), and all the power regulation circuitry (which appears to involve another three transistors) are closely arrayed across the top, walled in by 10 Nichicon and Rubycon through-hole electrolytic capacitors. These are all very high-quality components in an elegantly engineered package, and it's clear that Royer's boffins have designed the dBooster to the highest possible standard regardless of cost or effort (that's entirely predictable, given the quality of the company's microphones!).

Whereas most other manufacturers have gone down the 'less is more' route with their cascade preamp designs... Royer's backroom boys have really gone to town.

Bench Tests

Testing the dBooster using an Audio Precision analyser, I measured the two gain modes at 14.3dB (12dB setting) and 21.9dB (20dB mode), and testing a second unit provided exactly the same figures (within 0.1dB) so there shouldn't be any issues with gain-matching stereo pairs. The gain also remained consistent over a wide range of typical preamp input impedances above 1.5kΩ, although the gains reduced to 5.5dB and 17.9dB, respectively, when driving a vintage 600Ω destination. The dBooster's own input impedance is a comfortable 3.6kΩ, and I can confirm that phantom power is not present at the mic input socket.

This plot reveals excellent total harmonic distortion levels at the two gain settings.This plot reveals excellent total harmonic distortion levels at the two gain settings.The frequency responses at the two gain settings are very flat (the purple line is the input signal).The frequency responses at the two gain settings are very flat (the purple line is the input signal).Looking at the frequency response plots, these are ruler flat from 12Hz to over 80kHz, with a low-end -3dB point at 7Hz, for both gain settings. THD plus noise measured 0.02 percent with an input level of -50dBu, while a SMPTE intermodulation distortion test reported around 0.002 percent with a fixed -30dBu input at 7kHz and a swept tone from 1kHz down to 40Hz.

I found the input stage overloaded at -24dBu when in the 20dB setting, and -5dBu for the 12dB option, so there is masses of headroom available in typical applications, and although the second and third harmonics started to rise quickly over a couple of decibels below those clipping levels, the dBooster actually sounds exceptionally clean and transparent, with no hint of increasing edginess or transient distortion until just prior to hard clipping — that's impressive. My calculations of the EIN (equivalent input noise) figures are -120dB for the 20dB setting and -129.5dB for the 12dB setting (ref 1dB below clip levels), which are excellent for a low-gain device of this type. The output provides a fully symmetrical signal, with both legs driven actively.

In comparison with the sE Electronics Dynamite that I measured earlier this year (https://sosm.ag/se-dm1-dynamite), I found the dBooster's residual noise floor was around 8dB lower, which is another impressive result. Distortion was also less overall, and while the Dynamite clipped 6dB higher (at -18.5dBu) than the dBooster in its 20dB mode, selecting the 12dB gain mode pushed the dBooster's clipping level 12dB higher than the Dynamite's.

Conclusion

I tested the Royer dBooster mostly with gentle spoken voice into a Royer R10 placed about nine inches away, feeding a GML 8304 preamp. During my tests I adjusted the gain of the GML to compensate for the presence of the dBooster in each of its two gain settings, and then fine-tuned the gain matching in the DAW (as the GML switches gain in 5dB steps). With the connection between dBooster and GML via a single 3m XLR cable it was actually quite hard to tell whether the dBooster was in place or not, which I think is to its great credit. So, to give it a real workout, I compared the signal directly from the R10 down a 50m cable reel, and then with the dBooster driving the cable reel. There was a noticeable difference when the dBooster was connected between the mic and cable reel, with the sound becoming more open, with better transients and improved high-end extension. Although hard to be sure given the ambient room noise, I'd like to think the noise floor improved slightly too. I also tested the dBooster with the Shure SM7b, as well as pairs of AEA R92 ribbons and vintage AKG D224e dual-capsule microphones on a variety of gentle sources, and enjoyed excellent results in each case.

Overall, I found the dBooster to be a very fine mic booster indeed. It's about as transparent as it's possible to be, with a very quiet noise floor and extremely clean and neutral sound character. The dBooster takes a far more elaborate approach to the task than most cascade preamps, and consequently costs rather more. But it is also more versatile and performs exceptionally well. I've not measured many cascade preamps, but I have used a variety of them and the dBooster is easily the best I've used to date. Very impressive indeed!

Pros

  • An exceptionally elegant and sophisticated design.
  • Four paralleled transistors process each leg of the balanced input.
  • Very low noise; extremely clean and transparent.
  • Active balanced output driven by high-quality op-amps.
  • 12 and 20 dB gain modes.
  • Isolates phantom power from microphone.
  • Powerful line-driving output stage.

Cons

  • With such sophisticated circuitry, the dBooster is inevitably more expensive than its peers.

Summary

Mic gain boosters have become very popular in recent years, and provide a very effective way of using insensitive dynamic microphones with low-gain budget mic preamps. However, Royer's take on the idea is characteristically elegant and extremely sophisticated, and delivers an exceptional level of performance.

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Published July 2019